Common solid soaps are normally produced by a framing method or a milling method by using fatty acid soap as the base and by adding sugars or polyols such as sucrose, glycerin, sorbitol, and propylene glycol as necessary.
The fatty acid counter ion has a large effect on the properties of soap. If sodium is used as the counter ion, the solidification point and the hardness normally increase, and it is easy to adjust the shape as solid soap. On the other hand, the solubility in cold water and the foaming property decrease, and they tend to decrease the cleansing power and the feeling in use. In contrast to this, if potassium is used as the counter ion, the solubility in cold water and the foaming property are largely improved. However, the solidification point and the hardness significantly decrease, and production suitability as solid soap and the shape-retaining property deteriorate. Therefore, potassium is widely used, as the counter ion, for liquid soap (liquid body soap etc.). However, the application of about 20% of potassium has virtually been the limit for solid soap.
Especially in transparent soap, it is necessary to add a substantial amount of sugars or polyols to achieve transparency. Thus, the decrease in the solidification point is large, and the use of potassium as the counter ion tends to be difficult.
That is, the structural mechanism of letting transparent soap be transparent is considered that opaque soap fibrous microcrystals, which are optically discontinuous in size with respect to visible light, are mainly severed perpendicularly to the fiber axes by the addition of the above-described sugars and polyols and refined to the size of a wavelength of visible light or less; as a result, the soap becomes transparent. Therefore, the hardness and the solidification point easily decrease compared with the soap in which sugars and polyols are not added.
In particular, when transparent soap is produced by the framing method without using ethanol as the solvent for sugars and polyols, cutting, shape forming, and packaging are often carried out immediately after the removal of the frame. Thus, the decrease in the solidification point and the decrease in the hardness also directly lead to the deterioration of production suitability.
Therefore, it has been difficult to use potassium, which tends to lower the hardness and solidification point, as the counter ion.
On the other hand, soaps in which amino acids or trimethylglycine is blended are publicly known (Japanese Unexamined Patent Publication No. 2001-40390 and WO2004/029190); however, the presence of adjustment effects for the decrease of the solidification point and the hardness, when a large amount of potassium is used as the counter ion, has been totally unknown.